function [out_image] = entropy_image(in_image)
% entropy_image replaces each pixel
% with the (estimated) number of bits
% necessary to encode that pixel.
% Usage:
%   cameraman =  imread('cameraman.tif');
%   c = entropy_image(cameraman);
%   figure; colormap(gray(256));
%   c2 = 255-(c-1)*(255/9);
%   c2 = imclip( c, 10, 2, 256 );
%   image(double(c2)+1); drawnow;
% % translate c=1 bit to white (255),
% % translate c=10 bits or more to black (0).
%
%   row_diff = ...
%     [ double(cameraman(:,1))  diff(double(cameraman'))' ];
%   r = entropy_image( row_diff );
%   figure; colormap(gray(256));
%   r2 = 255-(r-1)*(255/9);
%   image(double(r2)+1)
%   dd = entropy_image( diff2( cameraman ) );
%   figure; colormap(gray(256));
%   d3 = 255-(dd-1)*(255/9);
%   image(double(d3)+1)

% Change log:
% 1999-07-08:DAV: David Cary <d.cary@ieee.org> started.

% See also ENTROPY, ENTROPY_GRAPH.

if( isa( in_image, 'uint8' ) )
   x = in_image;
else,
   minimum = min(in_image(:)); 
   x = round(in_image - minimum);
end;
pixels = prod(size(x));
histogram = histo(x(:));
% find all values that occurred at least once
occurred = find(histogram);
% For each value that actually occurred,
% estimate how many bits it takes to
% encode that value.
P = histogram(occurred) ./ pixels;
bits = inf*ones(size(histogram));
bits(occurred)=-log2(P);
%bits(occured)=ceil( -log2(P) );
% bits(1) indicates how many bits each pixel with value "0" requires,
% were one to compress it with entropy coding.
% This is generally a good estimate of Huffman compression,
% except when bits(1) is very small.
% (If there were negative values in the original image,
% bits(1) indicates how many bits pixels with the most negative value requires).

% Create the output image via look-up table
out_image = zeros(size(x));
out_image(:) = bits(1+double(x));

% We're done !
% Consistency checking;
% ASSERT( never looked up pixels that we thought didn't exist )
if( isequal( [], find( inf == out_image ) ) ),
else,
   error( 'Sorry, internal programming error in entropy_map.' )
end;
% ASSERT( It's impossible for a pixel to be represented in zero bits, unless ...
%         the entire image is the same color. )
if( isequal( [], find( 0 == out_image ) ) ),
else,
   % All pixels are the same color as the first pixel, right ?
   if( isequal( [], find( x(1) ~= x ) ) ),
      disp(' the entire image is the same color ')
   else,
      error( 'Sorry, internal math error in entropy_map.' )
   end;
end;

% end entropy_image.m